16,255 research outputs found
A Look At Three Different Scenarios for Bulge Formation
In this paper, we present three qualitatively different scenarios for bulge
formation: a secular evolution model in which bulges form after disks and
undergo several central starbursts, a primordial collapse model in which bulges
and disks form simultaneously, and an early bulge formation model in which
bulges form prior to disks. We normalize our models to the local z=0
observations of de Jong & van der Kruit (1994) and Peletier & Balcells (1996)
and make comparisons with high redshift observations. We consider model
predictions relating directly to bulge-to-disk properties. As expected, smaller
bulge-to-disk ratios and bluer bulge colors are predicted by the secular
evolution model at all redshifts, although uncertainties in the data are
currently too large to differentiate strongly between the models.Comment: 19 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Enhanced sensitivity of postsynaptic serotonin-1A receptors in rats and mice with high trait aggression
Individual differences in aggressive behaviour have been linked to variability in central serotonergic activity, both in humans and animals. A previous experiment in mice, selectively bred for high or low levels of aggression, showed an up-regulation of postsynaptic serotonin-1A (5-HT1A) receptors, both in receptor binding and in mRNA levels, in the aggressive line. The aim of this experiment was to study whether similar differences in 5-HT1A receptors exist in individuals from a random-bred rat strain, varying in aggressiveness. In addition, because little is known about the functional consequences of these receptor differences, a response mediated via postsynaptic 5-HT1A receptors (i.e., hypothermia) was studied both in the selection lines of mice and in the randomly bred rats. The difference in receptor binding, as demonstrated in mice previously, could not be shown in rats. However, both in rats and mice, the hypothermic response to the 5-HT1A agonist alnespirone was larger in aggressive individuals. So, in the rat strain as well as in the mouse lines, there is, to a greater or lesser extent, an enhanced sensitivity of postsynaptic 5-HT1A receptors in aggressive individuals. This could be a compensatory up-regulation induced by a lower basal 5-HT neurotransmission, which is in agreement with the serotonin deficiency hypothesis of aggression.
Towards a fully self-consistent spectral function of the nucleon in nuclear matter
We present a calculation of nuclear matter which goes beyond the usual
quasi-particle approximation in that it includes part of the off-shell
dependence of the self-energy in the self-consistent solution of the
single-particle spectrum. The spectral function is separated in contributions
for energies above and below the chemical potential. For holes we approximate
the spectral function for energies below the chemical potential by a
-function at the quasi-particle peak and retain the standard form for
energies above the chemical potential. For particles a similar procedure is
followed. The approximated spectral function is consistently used at all levels
of the calculation. Results for a model calculation are presented, the main
conclusion is that although several observables are affected by the inclusion
of the continuum contributions the physical consistency of the model does not
improve with the improved self-consistency of the solution method. This in
contrast to expectations based on the crucial role of self-consistency in the
proofs of conservation laws.Comment: 26 pages Revtex with 4 figures, submitted to Phys. Rev.
The small protein CydX is required for function of cytochrome bd oxidase in Brucella abortus.
A large number of hypothetical genes potentially encoding small proteins of unknown function are annotated in the Brucella abortus genome. Individual deletion of 30 of these genes identified four mutants, in BAB1_0355, BAB2_0726, BAB2_0470, and BAB2_0450 that were highly attenuated for infection. BAB2_0726, an YbgT-family protein located at the 3' end of the cydAB genes encoding cytochrome bd ubiquinal oxidase, was designated cydX. A B. abortus cydX mutant lacked cytochrome bd oxidase activity, as shown by increased sensitivity to H(2)O(2), decreased acid tolerance and increased resistance to killing by respiratory inhibitors. The C terminus, but not the N terminus, of CydX was located in the periplasm, suggesting that CydX is an integral cytoplasmic membrane protein. Phenotypic analysis of the cydX mutant, therefore, suggested that CydX is required for full function of cytochrome bd oxidase, possibly via regulation of its assembly or activity
Relativistic Brueckner-Hartree-Fock calculations with explicit intermediate negative energy states
In a relativistic Brueckner-Hartree-Fock calculation we include explicit
negative-energy states in the two-body propagator. This is achieved by using
the Gross spectator-equation, modified by medium effects. Qualitatively our
results compare well with other RBHF calculations. In some details significant
differences occur, e.g, our equation of state is stiffer and the momentum
dependence of the self-energy components is stronger than found in a reference
calculation without intermediate negative energy states.Comment: 13 pages Revtex, 5 figures included seperatel
Star formation in disk galaxies driven by primordial H_2
We show that gaseous \HI disks of primordial composition irradiated by an
external radiation field can develop a multiphase medium with temperatures
between 10^2 and 10^4 K due to the formation of molecular hydrogen. For a given
\HI column density there is a critical value of the radiation field below which
only the cold \HI phase can exist. Due to a time decreasing quasar background,
the gas starts cooling slowly after recombination until the lowest stable
temperature in the warm phase is reached at a critical redshift .
Below this redshift the formation of molecular hydrogen promotes a rapid
transition towards the cold \HI phase. We find that disks of protogalaxies with
10^{20}\simlt N_{HI}\simlt 10^{21} cm^{-2} are gravitationally stable at
K and can start their star formation history only at z \simlt
z_{cr}\sim 2, after the gas in the central portion of the disk has cooled to
temperatures T\simlt 300 K. Such a delayed starbust phase in galaxies of low
gas surface density and low dynamical mass can disrupt the disks and cause them
to fade away. These objects could contribute significantly to the faint blue
galaxy population.Comment: 16 pages (LaTeX), 2 Figures to be published in Astrophysical Journal
Letter
Color Gradients and Surface Brightness Profiles of Galaxies in the Hubble Deep Field-North
We fit elliptical isophotes to the Hubble Deep Field-North WFPC-2 and NICMOS
data to study the rest-frame UV_{218}-U_{300} color profiles and rest-frame B
surface brightness profiles of 33 intermediate redshift galaxies (0.5 <= z <=
1.2) with I_{814} < 25 and 50 high redshift galaxies (2.0 <= z <= 3.5) with
H_{160}< 27. From the weighted least-squares fit to the color profiles we find
that, at intermediate redshifts, the galaxies possess negative color gradients
indicating a reddening towards the center of the profile similar to local
samples whereas, at high redshifts, the galaxies possess positive color
gradients. This indicates that star formation is more centrally concentrated in
the distant galaxy sample which differs from the prevalent mode of extended
disk star formation that we observe in the local universe. Additionally, we
find that it is critical to correct for PSF effects when evaluating the surface
brightness profiles since at small scale lengths and faint magnitudes, an
r^{1/4} profile can be smoothed out substantially to become consistent with an
exponential profile. After correcting for PSF effects, we find that at higher
look-back time, the fraction of galaxies possessing exponential profiles have
slightly decreased while the fraction of galaxies possessing r^{1/4} profiles
have slightly increased. Our results also suggest a statistically insignificant
increase in the fraction of peculiar/irregular type galaxies. We compare our
results with recent semi-analytical models which treat galaxy formation and
evolution following the cold dark matter hierarchical framework.Comment: 31 pages, 10 JPEG figures. To be published in AJ Vol. 124, October
200
The Stellar Populations of Low Surface Brightness Galaxies
Near-infrared (NIR) K' images of a sample of five low surface brightness disc
galaxies (LSBGs) were combined with optical data, with the aim of constraining
their star formation histories. Both red and blue LSBGs were imaged to enable
comparison of their stellar populations. For both types of galaxy strong colour
gradients were found, consistent with mean stellar age gradients. Very low
stellar metallicities were ruled out on the basis of metallicity-sensitive
optical-NIR colours. These five galaxies suggest that red and blue LSBGs have
very different star formation histories and represent two independent routes to
low B band surface brightness. Blue LSBGs are well described by models with
low, roughly constant star formation rates, whereas red LSBGs are better
described by a `faded disc' scenario.Comment: 5 pages LaTeX; 2 embedded figures; MNRAS Letters, Accepte
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